Field of the Application
The present application relates to a semiconductor optical element and a surface-emitting semiconductor optical element.
Background Art
A distributed feedback laser element (DFB-LD) having a complex coupling type of diffraction grating provided in a resonator, as described in Japanese Patent Laid-Open No. 2002-299758 or 5-29705 is known. In Japanese Patent Laid-Open No. 2002-299758, refractive-index-coupling-type, gain-coupling-type and complex-coupling-type DFB lasers are contrasted with each other, as described below. In a refractive-index-coupling-type DFB laser, only the real part of a complex refractive index changes periodically in a resonator. That is, this type of laser is a semiconductor laser element having in a resonator a diffraction grating having the refractive index periodically changed. In a gain-coupling-type DFB laser, only the imaginary part of a complex refractive index changes periodically in a resonator. That is, this type of laser is a semiconductor laser element having in a resonator a diffraction grating having a gain periodically changed or having a loss periodically changed. A complex-coupling-type DFB laser is a type of semiconductor laser element in which both the real part and the imaginary part of a complex refractive index change periodically in a resonator.
A gain-coupling-type diffraction grating includes a light-absorbing layer and therefore has a problem of the threshold current being increased, a problem of the slope efficiency being reduced and a problem of the power consumption being high. In a case where a refractive-index-coupling diffraction grating is used, these problems can be avoided.
In a process of manufacturing a DFB laser, a diffraction grating is formed by etching, a diffraction grating embedding layer is thereafter grown and a light confinement layer, and active layer or the like is provided on this diffraction grating embedding layer. If in this process the diffraction grating is formed deeper than that specified with a design value due to manufacturing variation, the distance between the diffraction grating and the light confinement layer, the active layer or the like is smaller than the target design value. In this case, there is a problem of an optical coupling coefficient becoming larger than the target design value. For example, according to one of methods for avoiding this problem, the thickness of the diffraction grating embedding layer may be changed with reference to the diffraction grating depth so that the distance between the diffraction grating and the light confinement layer equals the design value. This method, however, entails a problem that the manufacturing efficiency is reduced.
In a surface-emitting semiconductor optical element, a periodic structure layer which periodically changes the refractive index along planar directions is provided. A variation in etching depth can occur with this periodic structure layer, as in the case of the diffraction grating, resulting in occurrence of an in-plane variation in optical coupling coefficient.